In its most usual form, a robot or manipulator comprises a series of arm segments terminating in a wrist assembly. The wrist assembly supports a face plate to which an appropriate tool is affixed. The nature of the tool will depend upon the work to be performed by the manipulator.
The manipulator is provided with a control. The control is programmed to cause the manipulator to move the tool along a programmed path of travel and to execute programmed functions associated with programmed locations along that path of travel.
In order for the manipulator to perform operations correctly, it is necessary that the manipulator move the designated center point of the tool accurately along the programmed path of travel. Some operations require greater accuracy than others. Examples of operations requiring particular accuracy include sealant application, contour mapping, deburring and welding. While not intended to constitute a limitation, the present invention, for purposes of an exemplary showing, will be described in its application to welding operations.
Once a manipulator is taught a program, it will move along the programmed path with high repeatability. While this is satisfactory for many applications, those operations of the type mentioned above and needing particular accuracy will require adaptive path control. For example, in a welding operation, if the parts being joined are not uniform in seam or joint location by virtue of manufacturing tolerances, heat distortion, shifting of the clamping fixture for the parts, or the like, the manipulator will weld along its taught path, even though the seam may be located elsewhere.
Prior art workers have devised a number of systems for adaptive path control. Each system involves some sort of sensor. One system constitutes a pendant controlled root or initial pass wherein the operator creates deviations from the taught path of manipulator arm travel by eye and through the use of a pendant. This system is relatively slow and depends on the skill of the operator. A forced torque pressure tracking technique is known in the art, as well as a through-the-arc seam tracking system requiring that the seam be characterized by opposing sidewalls which are contacted by the tool center point (i.e., the tip of the welding tool) which traverses a weaving path. This weaving tracking imposes limitations on the welding speed. Yet another system is taught in copending application serial number 07/179,505, filed Apr. 8, 1988 in the names of Gregory Webb and Keith R. Wehmeyer and entitled VISION SEAM TRACKING METHOD AND APPARATUS FOR A MANIPULATOR. This system employs a laser scanner/camera head which previews the seam to be welded ahead of the welding torch. Data from the vision system is used to alter the manipulator's preprogrammed path of travel.
A problem which plagues the welding industry lies in the fact that many welding operations require successive weld passes to be made on the same seam. Thus, each pass must be seam-tracked, adding to the weld time required. In many instances seam-tracking cannot be performed over a weld bead. In addition, applications exist wherein the space provided will not accommodate the tracking equipment and the welding torch at the same time. Thus, a method and means are needed whereby the tracking equipment alone can be used to do an initial dry run or root pass determining the proper offset path of travel, followed by successive passes with the welding torch only, performing the welding operation based on the data observed in the initial seam-tracked pass. Even in those instances where no space problem exists, if an initial seam-tracked pass could be made (with or without concurrent welding), followed by successive welding passes without seam-tracking and based on the data observed in the seam-tracked pass, the multi-pass welding operation could be simplified, accomplished more quickly, and performed with less chance of damage to the seam-tracking equipment.
The present invention is based upon the development of a new feature enabling the manipulator operator to memorize the offsets generated during an initial seam-tracking pass, controlling the rate at which the offsets are accumulated. Thereafter, without the use of a seam-tracking system, the operator can recall the offsets on successive passes executed in the same direction as the initial seam-tracked pass or in a direction opposite to that of the seam-tracked pass. Thus, for example, alternate passes can be executed in the opposite direction, if desired. This new feature, with the aid of existing taught point offset registers (or the equivalent) enables the manipulator to perform multipass welding requiring some form of seam-tracking only on the initial or root pass. The present invention provides the manipulator operator with an exceptional degree of flexibility for both memorizing and recalling the offsets generated either manually or by means of a sensor or seam-tracking system.